In recent years, for meeting demands for saving space, simplifying assembling work, improving control performance and the like, more and more mechanical drive systems are replaced with by-wire systems, which are driven by electric motors.
In a by-wire system disclosed in JP-A-2006-336691 (US 2006/0271260 A1), a monitor control circuit such as an engine control circuit is provided separately from a by-wire control circuit, which controls a motor provided as a drive power source. This monitor control circuit monitors the by-wire control system based on predetermined monitor information. When the monitor control circuit detects abnormality of the by-wire system, it prohibits the by-wire control circuit from controlling the motor by, for example, prohibiting power supply to the motor by turning off a switch device.
According to this technology, the power supply to the motor is prohibited by turning off the switch device when the abnormality of the by-wire system is detected. In this case, even when the power supply to the motor is shut down under a state that the motor is in rotation, the motor is likely to continue to rotate by inertia of the motor or the like. As a result, it is not possible to stop quickly the mechanical operation of the by-wire system at the time of the abnormality of the by-wire system. It is thus likely that safety of the by-wire system cannot be ensured at the time of occurrence of abnormality.